1. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for variable rate communication incorporating coherent signal processing.
2. Description of the Related Art
FIG. 1 is an illustration of a wireless cellular telephone system configured in accordance with the IS-95 over-the-air interface standard (The IS-95 standard). The IS-95 standard has been adopted by the Telecommunications Industry Association (TIA) and utilizes code division multiple access (CDMA) modulation techniques to provide greater capacity and more robust performance over prior art wireless telecommunications technologies. In accordance with the IS-95 standard, subscriber units 10 (usually cellular telephones) establish bi-directional links with one or more base stations 12 via the use of radio frequency (RF) electromagnetic signals in order to conduct mobile telephone calls or other communications. Each bi-directional link consists of a forward link RF signal transmitted from a base station 12 to a subscriber unit 10 and a reverse link RF signal transmitted from a subscriber unit 10 to a base station 12. The telephone call or other communication is further processed from each base station 12 by way of mobile telephone switching office (MTSO) 14 and public switched telephone network 16 (PSTN), which are usually coupled to one another using wire line connections.
FIG. 2 is a block diagram of a reverse link transmit signal processing system employed by a subscriber unit 10 in accordance with the IS-95 standard. Data 48 is provided to convolutional encoder 50 in 20 ms segments, called frames, at one of four rates referred to as "full rate", "half rate", "quarter rate", and "eighth rate" respectively as each frame contains half as much data as the previous and therefore transmits data at half the rate. Data 48 is typically variable rate vocoded audio information where lower rate frames are used when less information is present such as during a pause in a conversation. Convolution encoder 50 convolutionally encodes data 48 producing encoded symbols 51, and symbol repeater 52 generates repeated symbols 53 by symbol repeating encoded symbols 51 by an amount sufficient to generate a quantity of data equivalent to a full rate frame. For example, three additional copies of a quarter rate frames are generated for a total of four copies while no additional copies of a full rate frame are generated. Block interleaver 54 then block interleaves the repeated symbols 53 to generate interleaved symbols 55. Modulator 56 performs 64-ary modulation on interleaved symbols 55 to produce Walsh symbols 57. That is, one of sixty-four possible orthogonal Walsh codes, each code consisting of sixty-four modulation chips, is transmitted for every six interleaved symbols 55. Data burst randomizer 58 performs gating, using frame rate information, on Walsh symbols 57 in pseudorandom bursts such that only one complete instance of the data is transmitted.
A timing diagram illustrating an exemplary gating performed by data burst randomizer 58 during the transmission of a frame of data is shown in FIG. 3. The time intervals associated with the transmission of a frame is divided into sixteen burst slots. Each burst slot is referred to as a "power control group" because a receiving base station typically makes a power strength measurement on each burst slot received in order to transmit power control information to the subscriber unit. In the exemplary embodiment shown, data is transmitted during all sixteen power control groups for a full rate frame and during power control groups 0, 2, 5, 6, 9, 11, 12, and 15 for a half rate frame. For a quarter rate frame data is transmitted during power control groups 2, 5, 9, and 15, and during an eighth rate frame data is transmitted during power control groups 2 and 9. This is just a set of exemplary gatings. In accordance with the IS-95 standard, the repetition performed by symbol repeater 52 and the interleaving performed by block interleaver 54 are such that the gating of the data as described above causes one instance of the data in the frame to be sent.
The gated Walsh chips are then direct sequence modulated using a pseudorandom (PN) long channel code 59 at rate of four long channel code chips to each Walsh chip generating modulated data 61. The long channel code is unique for each subscriber unit 10 and is known by each base station 12. Modulated data 61 is duplicated with the first copy being "spread" via modulation with an in-phase pseudorandom spreading code (PN.sub.I) producing I-channel data, and the second copy is delayed one half a spreading code chip by delay 60 and spread via modulation with a quadrature-phase spreading code (PN.sub.Q) producing Q-channel data. The PN.sub.I code and PN.sub.Q code spread data are each low pass filtered (not shown), before being used to modulate in-phase and quadrature-phase carrier signals respectively. The modulated in-phase and quadrature-phase carrier signals are then summed together before being transmitted to a base station or other receive system (not shown).
FIG. 4 is a block diagram of a receive processing portion of a base station when configured in accordance with prior art methods for receive processing a reverse link signal generated in accordance with the IS-95 standard. The processing shown is consistent with that described in U.S. Pat. No. 5,442,627 issued Aug. 15, 1995. Other patents related to prior art methods for receive processing include U.S. Pat. Nos. 5,103,459 and 5,309,474, both entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM" and issued Apr. 7, 1992 and May 3, 1994 respectively, as well as U.S. Pat. No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM" issued Apr. 28, 1992. Each of the above referenced patents are assigned to the assignee of the present invention and incorporated herein by reference.
During operation, the IS-95 reverse link signal is received from antenna system 64 and downconverted to baseband and digitized by RF processing system (Rx) 62, and the downconverted signals are applied to finger processors 63A, 63B, and 63C. As shown in the more detailed depiction of finger processor 63A, demodulator 66 demodulates the downconverted signal using the PN.sub.I code and PN.sub.Q code respectively, and sums the result of that demodulation for every four PN chips to generate I-channel and Q-channel data 68A and 68B. The I-channel and Q-channel data are then applied to Walsh transformer circuits 69 and 70 respectively, each of which correlates the despread data with the sixty four available Walsh codes, thereby generating a vector of sixty-four correlation values corresponding to the sixty four available Walsh codes for both the I-channel and Q-channel data. Each of the two vectors are then squared by squaring circuits 72, and the resulting two vectors of squared data are summed by summer 74. After the introduction of variable delay (not shown) to adjust for path differences, the vector of squared correlation values from summer 74 is summed with the other sets of squared correlation values generated by finger processors 63B and 63C by summer 76. Finger processors 63B and 63C are processing multipath instances of the same reverse link signal, if such multipath signals are available. The resulting vector of squared correlation values from summer 76 is then used to form soft decisions for each of the six symbols corresponding to a Walsh symbol sequence. These soft decisions are deinterleaved and Viterbi decoded to obtain estimates of the transmitted data. Various schemes for performing the soft decisions are described in the above referenced ('No. 627) patent.
The above described method and apparatus for receive processing system data employs noncoherent demodulation. The use of noncoherent demodulation is generally well suited for processing an IS-95 reverse link signal since no pilot signal is provided for determining the phase of the reverse link signal and the energy level of the data is kept at the minimum necessary to allow for successful communication. Additionally, noncoherent receive processing is generally less complex than coherent receive processing. However, noncoherent demodulation provides reduced performance when compared to coherent processing, including a reduction in the gains achieved by employing Rake receivers in which multipath instances of the reverse link signal, referred to as fingers, are summed together at the receive system as described above. This reduction in the benefits achieved from Rake reception makes it necessary for a subscriber unit to transmit the reverse link signal with additional power relative to that necessary if coherent signal processing were employed. Within an interface-limited system such as CDMA, the use of additional power reduces the overall data carrying capacity of the reverse link, and therefore the total number of calls that can be conducted. If, however, a method for coherently processing and combining a reverse link signal generated in accordance with the IS-95 standard could be devised, the required transmit power of a reverse link signal could be reduced, and thus the reverse link capacity of an IS-95 or other CDMA telecommunication system could be enhanced. Therefore, such a method and apparatus would be highly desirable.